This invention relates to a method of forming a high-quality print image by electrophotography using an adhesive transfer process. More particularly, this invention relates to a method in which a toner image formed on the photoconductor on a photoreceptor drum is first adhesive-transferred onto an intermediate transfer element having an adhesive layer and then retransferred completely onto a final support. This method is suitable for use in medical diagnostic imaging (e.g. ultrasographic imaging, X-ray imaging and MRI), the production of printing monochromatic and color proofs, as well as in the formation of print image of high contrast and quality by recording means such as a laser printer.
An image recording method is known in which a latent electrostatic image is formed on a uniformly charged photoconductor by illumination with a light beam modulated with an image signal carrying continuous-tone image information and a toner image that is subsequently formed by a conventional electrophotographic process is transferred onto a support to produce a hard copy of the image of interest. In order to obtain a hard copy by electrophotography, the toner image on the photoconductor must be finally transferred onto a support such as a sheet of paper and to meet this need, various methods of transfer have been proposed.
The most common method for transferring the toner image on the photoconductor onto a support such as a sheet of paper is "electrostatic transfer" in which the toner image is transferred electrostatically, for example, using a corotron. However, using this transfer method to form a continuous-tone image has presented the following problems. 1. The efficiency of toner transfer depends on the density of toner image and the toner on the photoconductor cannot be completely transferred. The efficiency of transfer is particularly low in high-density and low-density areas, so if the electrostatic transfer method is adopted to produce a cotinuous-tone image of high contrast, both the highlights or the gradation in the high-density areas will be lost.
Since not all of the toner particles on the photoconductor can be transferred onto the support, the residual toner on the photoconductor must be removed by various methods including wiping with a blade. But this increases the chance of damaging the surface of the photoreceptor to prevent the production of a high-quality continuous-tone image since any surface damage is prone to cause artificial images such as streaks and uneven densities. 2. When the toner image is to be transferred onto the support such as a sheet of paper, the efficiency of toner transfer is much influenced by the electrical properties of the support at the microscopic level to cause unevenness in the density of the final image. Further, the electrical properties of the support can fluctuate under varying environmental conditions and this makes it difficult to obtain a consistent image. Particularly in the case where the toner image is transferred onto a receiving sheet that has absorbed moisture, the transfer efficiency usually drops to cause marked deterioration in image quality.
Thus, it has been difficult for the conventional electrostatic transfer method to permit the toner image on the photoconductor to be completely transferred onto the final support so as to produce a continuous-tone image of high quality and contrast.
Another transfer method known in the art involves the use of an intermediate transfer element.
An example of this method of transfer is illustrated in FIG. 5. A toner image T on a photoconductor 114 on a photoreceptor drum 112 is not directly transferred onto a support such as a sheet of paper but is first transferred onto an intermediate transfer element 116 such as a silicone belt or a silicone rubber roll, from which the toner image T is retransferred onto the support such as a sheet of paper, typically by application of heat and/or pressure, to thereby obtain a hard copy. If desired, a corona discharge 118 may be applied to the back side of the support. In this method, the transfer of toner image from the photoconductor 114 onto the intermediate transfer element 116 relies basically upon the inherent tendency of the toner particles to adhere to the transfer element such as a silicone rubber roll but the force of their adhesion is generally insufficient to achieve high transfer efficiency. As a result, the toner is not completely transferred onto the transfer element and part of it will remain on the photoconductor 114. The residual toner must be wiped off with a blade 120 but then the same problems that are described above in connection with the conventional electrostatic transfer method will arise. Further, the retransfer of toner image T from the intermediate transfer element 116 onto the support such as a sheet of paper is also incomplete and the transfer efficiency is highly variable depending on electrical properties of paper at the microscopic level or on environmental conditions. As a result, the toner image transferred onto the support such as a sheet of paper is so much uneven in density that it does not cause any problems in producing binary-level images such as characters and line images but not suitable for the case where high fidelity of tone reproduction is required as exemplified by the production of high-contrast continuous-tone image. Under the circumstances, an "adhesive transfer method" which permits the toner image on the photoconductor to be transferred onto an adhesive layer has been proposed as a process that is effective in enhancing and stabilizing the efficiency of toner transfer.
An electrophotographic process that uses the adhesive transfer method for recording continuous-tone image with satisfactory tone reproduction has been disclosed in commonly assigned Japanese Patent Publication (Kokoku) No. 38172/1974. In this process, a latent electrostatic image formed on a photoconductor is developed with a liquid developer (hereinafter sometimes referred to as "a liquid toner") to form a toner image, onto which a sticky (adhesive) tape is compressed and thereafter peeled to separate the toner image, with the peeled tape being subsequently bonded to the final support. The liquid toner comprises fine charged toner particles dispersed in a dielectric fluid. The size of the toner particles is usually in the ragen of 0.1-1.0 .mu.m which is smaller than in dry developers and, therefore, the liquid toner is advantageous for the purpose of recording a continuous-tone image.
When color image is formed by this process with 3 - or 4-colored toner image being transferred onto a single sheet of adhesive tape, the efficiency of toner transfer decreases for the second and subsequent colors. In order to solve this problem, the assignee has proposed improved methods in commonly assigned Japanese Patent Application Nos. 299167/1986 and 73750/1987. According to those methods, a toner image of the first color is transferred onto a single sheet of adhesive tape, which is then attached to a single support for 3 or 4 colors. These methods are capable of recording color image of high quality and high contrast but, on the other hand, the largeness of the tape thickness imparts unnatural appearance to the final image.
With a view to solving this problem (the adhesive tape for adhesive transfer of toner image is so thick as to make the final image appear unnatural to the viewer), the assignee proposed improved methods in commonly assigned Japanese Patent Application (Kokai) Nos. 253760/1989, 253756/1989, 253757/1989 and 110587/1990. Those methods permit the use of an extremely thin adhesive tape for obtaining satisfactory toner image. According to those methods, a continuous-tone image can be the transfer recorded with fairly good tone reproduction; efficiency is close to 100% but it is still insufficient to produce a continous-tone image of extremely high contrast and quality.
Two other methods have so far been proposed for the purpose of achieving complete transfer of the toner image. The first method has been disclosed in Japanese Patent Application (Kokai) Nos. 174557/1986, 212668/1987 and 4261/1988 and is named a "signature color proofing system". As shown in FIG. 6a, this method is characterized by using a PC film 135 comprising a polyethylene terephthalate (PET) base 132 which has formed thereon a transparent conductive layer 133 serving as the ground, which in turn is coated with a light-sensitive OPC (organic photoconductor) layer 130 serving as a photoreceptor and then with an overcoat (OC) layer 131 made of a thermoplastic resin, with the other side of the PET base being provided with a matted backing layer 134. When the PC film 135 is electrified with a corona discharge device, the transparent conductive layer 133 is uniformly charged to negative polarity whereas the OC layer 131 on the light-sensitive OPC layer 130 is uniformly charged to positive polarity (see FIG. 6b). In an imagewise exposure step, the PC film 135 is illuminated with light which is applied to the backing layer 134 through an original 136. The exposing light is admitted into the photosensitive layer 130, where charges are eliminated from the illuminated areas and are left only in the unilluminated areas. Upon development with a toner, the particles of toner T will be deposited as shown in FIG. 6c. Then, a support 137 such as a sheet of paper is thermocompressed onto the OC layer 131 carrying the toner image T and the PC film 135 is separated into two parts, one including the OC layer 131 and the other including the light-sensitive OPC layer 130, whereupon the toner image T is transferred onto the support 137 together with the OC layer 131. In a color process, the steps of corona charging, exposure and toner development are repeated for the respective colors of interest to form toner images of predetermined colors, say, Y, M, C and B, on the OC layer 131, which toner images are then thermocompressed onto the support. Subsequently, the PC film 135 is separated into two parts as described above and the color toner images are transferred onto the support together with the OC layer 131, thereby forming a desired color image.
This is indeed a method capable of achieving a transfer efficiency of 100%. However, the need to strip the PC film from the light-sensitive layer in each step of transfer makes it impossible to use the photoreceptor repeatedly and the processing cost per hard copy becomes unavoidably high.
The second method has been disclosed in Japanese Patent Application (Kokai) No. 180248/1986 and is named a "LANDA process". This method is characterized by using the particles of a "tentacle toner" which comprises a plurality of fibers having a certain specific morphology. When the latent electrostatic image on the photoconductor on a photoreceptor drum is developed with a liquid toner having the particles of "tentacle toner" dispersed therein, the fibers are physically intertwined, interlocked or interconnected within the developed image as shown in FIG. 7a, thereby forming a dense image. In the transfer step, a chain of toner (T) particles are transferred as shown in FIG. 7b and it is expected that the toner image can be completely transferred from the photoreceptor drum 140 to the support 142 such as a sheet of paper. As a result, there is high possibility for obtaining a hard copy that carries a toner image T having high resolution and marked contrast.
The toner particles to be used in the second method desirably have such a nature that they will not agglomerate in a dispersion medium composed of a dielectric carrier fluid but that the concentration of those toner particles on the photoconductor will increase only when the toner image is formed, thereby permitting the toner particles to be interlocked or interconnected with a stronger force. In practice, however, it is difficult for one and the same toner particle to possess those incompatible properties. Hence, those toner particles have an inherent tendency to agglomerate in the dispersion medium, which presents considerable difficulty in handling the toner since the agglomerated toner particles are highly prone to settle. In other words, the toner particles described above have a great tendency to settle when they are not in use and in order to use them, the toner particles must be thoroughly redispersed by suitable means such as agitation.